Each new word until strong entry into colloquial use is considered a neologism. The absent earlier verbal new growths and also trends of their development and promoting are studied by the special section of linguistic science - a neologiya. So it neologisms and for what they are necessary to language?
The neologism is the term having the Greek origin, which is literally translated as "a new word". With its help it is accepted to designate words or their combinations which appeared in language recently. As a rule, in the developed languages annually there is a huge number of new word formations, the majority of which, however, are not set in colloquial use. Only limited quantity of neologisms become habitual for the people over time and receive the broad use, passing from passive lexical structure into active. New language word formations are necessary for enrichment of dictionary structure during the concrete historical period. Evolutionary progress does not stand still, and in each country new things and concepts, professions, technical devices, the cultural phenomena constantly appear. So there are lexical neologisms. Sochetayemostny neologisms are called turns in which interrelations of words with each other are unusual. When the unusual value is attributed to an old form of a word, speak about a semantic neologism. Besides language, language is regularly replenished with author's or individual neologisms. The important feature of such language new growths is that their uniqueness is not erased eventually. There are several algorithms of emergence of words neologisms in language: 1) the way of a word-formation derivation representing emergence of new growths from the morphemes existing in language on productive models; 2) the method of a semantic derivation meaning formation in already existing word of secondary value; 3) loan of foreign words; 4) in the way of introduction to language of prostorechiya, dialectic and slangy expressions. You should not confuse words neologisms with nonce words. The nonce word is a word which was formed in special conditions of speech communication. It often contradicts language norm and appears in the speech by means of a language game. Thus, emergence in language of neologisms is an indispensable condition of its natural development and expansion.